The invention relates to a sludge bulking predetermining method by an activated sludge process in a sewage treating process, comprising the following steps: establishing a sludge bulking symptom database; analyzing key off-line data by means of the expert knowledge; simply classifying the current working condition; carrying out corresponding treatment aiming at different working conditions, wherein, the sludge bulking symptom database establishment comprises the primary data set collection; recording the stored process history data through an automatic control system to obtain the primary data set of the sludge bulking symptom database; determining the structure of the database and a sludge bulking symptom attribute; analyzing the sewage treating process data by means of the mastered expert knowledge; and utilizing on-line process data which can dynamically reflect the sludge bulking symptom information to serve as the sludge bulking symptom attribute. By combining the expert knowledge and a data drive method, the method of the invention has the advantages that qualitative data, quantitative data, the off-line data and the on-line data can be comprehensively analyzed, the sludge bulking can be accurately predetermined, the occurrence frequency of the sludge bulking is reduced, the safe and stable operation of a sewage treating system is ensured, and the system maintenance expense is lowered.